Hot Rods I HEARD (READ) THAT A 30" TALL TIRE KNOCKS OFF 5 POINTS ON YOUR REAR GEAR RATIO

Does the height of the rear tires really take away from your rear end ratio? I heard a 30' tall rear tire will take the rear end ratio 5 points. As in a 4:11 gear with 30" tires will become a 4:06 gear on the ground. Any truth to this? Bill

 

Depends what tire you are staring with...

 

http://www.tremec.com/calculadora.php

Put in you tire size now and then try it with a new size. But it does slow the rpms at the same speed vs smaller tires.

 

It's the roll out that changes. A 30" tire has almost an eight foot roll out so the taller the tire the fewer revolutions the ring gear has to make to go a given distance effectively lowering the ratio.

Sent from my SM-G920T using The H.A.M.B. mobile app

 

Let's say you have a 4.11 axle and 28" tires. If you go to 30" tires, multiply the original tire diameter by the gear ratio and divide the result by the new tire diameter. 28 x 4.11 / 30 = 3.836 so in that case, it gives you the equivalent of having the original 28" tire and a 3.84 gear.

Going the other way, if you change from a 28" tire to a 26" tire, you'd have 28 x 4.11 / 26 = 4.426 So the new 26" tire would give you the equivalent of keeping the original 28" and changing to a 4.43 gear.

If you play around with the numbers, you'd see that a change in tire diameter of 1/2" will give you an equivalent of around 5 points difference in the effective gear ratio. If a person is changing to a larger tire, chances are they're going to change by more than 1/2", so the effective ratio change is going to be a lot more than 5 points.

 

What???

 

Only with a full race cam, if you have a 3/4 race cam it only knocks 3.75 off your rear gear ratio.

 

And things can get even more interesting on a circle track car when dealing with stagger and a fixed rear axle ratio.

 

Yep, true dat, sometimes more. It changes your speedometer and gas mileage, too. It's basic physics; the principle of the lever applies to the radius of the tire. It's the poor man's "quick change rear end".

 

Sometimes tire size(diameters) is the easiest(or cheapest) way to change the effective gear ratio, especially when there are only a limited number of axle gear ratios available, especially in pre WWII commercial vehicles. Ford had only 2 bevel gear ratios in the 1-1/2 ton trucks(5.14:1 & 6.6:1), so if you want to use one today some kind of overdrive helps, but going to a larger OD tire is often necessary as well. For example: Ford 1-1/2 ton truck/5.14 : 1 gear ratio with original 6.00x20"(32"+/- dia) tires & you change to 6.50 x 20(35"+/- dia) tires, the ratio changes to an approximately effective 4.70: 1 ratio. When you add a .82 Overdrive(T-5 trans) it changes to an approximately effective 3.85 : 1, which is a very "streetable" final drive ratio, especially in a commercial vehicle. I hope this example helps clear-up the questions about tire size changes, especially for street use.( If you have a T-5 with a .69 Overdrive, the effective final drive would be approximately 3.25:1)

 

My 32 with a ford 302 / c4 trans / 3.31 gear and 27" tall tire at 70 mph I was at 3100 rpm, put a 31" tire on and rpm dropped to 2700 rpms.

 

Glad to see this one. I have a 33" tall tire with a 3.25 gear in my '32 with a 302/C4. Hoping for 2250 cruising RPM, might be close!
Thanks for sharing,
SPark

 

33''tire. 3.25 gear. Equals 2150 rpm @ 65 mph.

 

So it's no theory----It's pure physics----I was sure it was but had no information on how to figure it out. Thanks HAMB!! Bill

 

My '31 is SBF, C4, 3.25's and 29.5" tires. 2300 at 55mph. A bit better than the 3.89's I had in it originally and it didn't kill that much bottom end.

 

I did the math (hazards of being an engineer) with a bunch of gear ratios and tire sizes trying to decide. We raced dirt late models for many years and you were always figuring how to change RPM a few hundred to stay in the power band you wanted. It will usually be a little less RPM than you figure if you get any tire growth at all. I was shooting for 2200 at 70MPH, looks like it will be close.
SPark

 

The thing is, if the gear ratio is off enough that you feel you need to change it, then you're going to need to make a pretty big change in tire size to make a noticeable difference...and there are usually at least seven good reasons that the tire size you would need, won't work right on the car.

So don't expect RPM miracles from changing tires.

 

The old 64 chevy 3/4 we used for a wrecker was all wound up a 60MPH with 750x16 tires. We installed a set of 19.5 tires and it lowered rpm and helped with fuel economy. It didn't pull as well but it don't bother me to downshift.

 

This is actually for electric cars but may be quite a bit easier to plug and play with than the one 1oldtimer gave above. Results are the same but this may be simpler. It lets you put in 3 out of 4 variables and get the 4th/ all direct drive though and no input for O
D.

http://www.advanced-ev.com/Calculators/TireSize/

Example 30 inch tire 4.11 gears 60 mph = 2762 rpm.
a 29.6 inch tire with 4.06 gears would hit the same 2762 rpm at 60.

 

Love my Isky Dream Wheel. All the answers fit in your shirt pocket.